1. Field of the Invention
A need exists for purposes of homeland and military security for a system that is capable of neutralizing GSM wireless devices that are in close proximity to motorcades or military convoys (both “convoys” in the following) and consequently present an immediate security threat to the convoy. Such a neutralizing system must be able to dynamically detect and suppress such GSM wireless devices. A similar need also exists to suppress GSM wireless devices when conducting wide area static operations such as may be related to crowd control or search and seizure. The neutralizing system described herein achieves these objectives while minimizing both the required power consumption and interference with wireless devices that do not present immediate security threats. Such interference is termed in the following collateral interference.
2. Description of Related Art
Prior art techniques for jamming GSM wireless devices require substantial amounts of sustained power and create unintended collateral interference over potentially wide areas. At present the GSM standard provides for a maximum spectrum allocation of 230 MHz. A cell tower and the corresponding wireless device communicating with it can conceivably be operating anywhere in this allocated spectrum. Furthermore, in a dynamic environment such as a convoy, these signals will appear and disappear as a convoy moves. Without the benefit of a receiver that is designed to detect this class of signal it is impossible to predict when or where (in either location or spectrum) a threatening signal will arise and therefore the entire allocated spectrum must be attacked simultaneously and continuously.
The GSM standard provides for effective radiated power (ERP) as seen at the tower antenna of more than a kW concentrated in a 200 kHz band. Presuming that a convoy can pass within 100 yards of this tower and presuming a protection radius of 100 yards, the minimum ERP of the jamming signal concentrated in the same 200 kHz band would be, as a minimum equal to that of the tower, (further presuming a minimum jammer power to signal power ratio of 0 dB and neglecting any fading effects). Presuming that it is only necessary to attack either the forward or reverse links to the wireless devices, this halves the potential spectral coverage to 115 MHz (575,200 kHz channels). In this case, the necessary power will be the maximum potential tower power multiplied by 575 channels, which translates to several hundred kW if attacking the forward link. Reducing the tower ERP to reflect more typical levels of perhaps 50 watts requires a still formidable 29 kW of worst case jamming power before factoring in other effects such as jammer fading compensation.
Reverse link power requirements are more modest, as it is presumed that it is only necessary to overcome the power of the wireless device as seen at the tower. The standard provides for a maximum of 8 Watts with typical of 1 Watt corresponding to a total of about 5 kW and 625 Watts, respectively of sustained power using the 0 dB jammer to signal power ratio criterion established above. However, the actual power required to effect a reverse link attack may prove to be somewhat higher due to vagaries attributable to the relative geometry of the jammer with respect to the wireless device and the tower sectoring—for example when a jammer finds itself in a side-lobe vis-à-vis the tower and is attempting to overcome a wireless device that is directly in the beam (i.e., the jammer has poor tower visibility and the wireless device has good tower visibility). Because such geometry is difficult to predict, this can easily increase the required power may increase by a factor of 10 or more to account for such cases. Furthermore a reverse channel attack is typically not considered viable because                a) it can be blinded altogether to the tower due to antenna sectoring;        b) will likely be thwarted by a network attempt to handover to another tower;        c) the method of attack relies on the network dropping the call which can be on the order of 15 seconds (typical), during which the forward link (which is used to effect detonation) remains viable; and        d) will likely disrupt the communications of all of the subscribers operating on perhaps multiple towers and hence have an unintended collateral affect over potentially 10 s of square miles.        
After factoring in the power amplifier inefficiencies and other practical implementation losses, the overall power consumption can increase by another factor of between 2 to 4. The potential for overall power consumption and collateral interference therefore renders a continuous wideband attack useless for modes of operation in which there are limits on power consumption and collateral interference.
Many prior art techniques recognize this limitation and augment wide band jamming with a receiver that is capable of detecting when a wireless device is active within some prescribed radius. The jamming system then reacts to neutralize the active wireless device. These jamming techniques have the obvious advantage of only operating when a wireless device signal is detected and limiting transmission to that portion of the spectrum where the wireless device is signaling. This can, in principal, reduce the average overall power consumption by perhaps a factor of 100 or more. However because these techniques are indiscriminate with respect to the nature of the jamming signal waveform (e.g. using white noise over some non-tailored sub-band of the cellular spectrum); the fact that a GSM wireless device can employ frequency hopping over potentially wide swaths of spectrum; and in high subscriber density areas there can be constant wireless device detection and hence suppression, the promised power savings will likely not materialize or will be greatly diminished in many practical cases.
There are also other practical limitations associated with wideband jamming techniques. First, the presumption above is that detection is limited to looking for reverse link signaling by the wireless device as it will be in close proximity to the jammer and therefore will be the most prominent discriminating factor. This introduces timing conflicts because the system is attempting to jam a signal while it is listening for the same signal. The system cannot do both simultaneously because transmitting while receiving will necessarily blind the receiver and likely damage it. This limitation demands that the transmitter and receiver must time duplex, increasing the risk that a signal gets through to the wireless device while the system is listening or perhaps a new signal is not detected in a timely fashion for the same reason. This method also makes it challenging to determine when to end the attack because the receiver may have difficulty determining whether the lack of signal is due to the end of wireless device signaling or due to being effectively blinded by the transmitter.
Problems not solved by known techniques are: the inability to selectively and efficiently filter for access only approved wireless devices; the inability to react in a timely fashion to the dynamic conditions in an operational area; the inability to limit collateral interference to an operational area; and the inability to tailor the suppression so as to achieve minimum power consumption while being maximally inconspicuous.
It is an object of the neutralizing systems disclosed herein to solve these and other problems related to access filtering and threat suppression thereby to provide improved techniques for access filtering and threat suppression. In the following, these improved techniques will be termed surgical access filtering and threat suppression techniques. A system that employs such techniques to neutralize wireless devices will be termed a surgical neutralizing system.